Determination of the intracellular signaling pathways that control intestinal epithelial proliferation is fundamental to the understanding of the integrity and function of the intestinal tract under normal and diseased conditions. The phosphatidylinositol 3-kinase (PI3K)/Akt pathway transduces signals initiated by growth factors and is involved in cell proliferation, cell differentiation, and programmed cell death. Our in vivo and in vitro studies demonstrate that PI3K transduces mitogenic signals initiated by growth factors, regulates expression of important cell cycle-related proteins, and controls intestinal epithelial proliferation. Specific Aim 1 of this application is to determine the critical role of PI3K in intestinal epithelial proliferation using animal models in which gut mucosal proliferation will be induced by growth factors and small bowel resection. Intestinal localization of PI3K activity, cyclin D1 protein, and epithelial proliferation will be evaluated. The second Specific Aim will delineate the molecular mechanisms by which PI3K and its downstream effectors regulate intestinal epithelial cell proliferation. The regulation and functional role of cyclin D1 and p27/Kip in PI3K-mediated intestinal epithelial proliferation will be elucidated. In Specific Aim 3, we will determine whether functional gain in PI3K improves intestinal mucosal proliferation employing knockout mice and transgenic mice in which PI3K activity is functionally upregulated. Regulation of intestine-specific genes by PI3K activation will be investigated in these animal models. The long-term goal of this proposal is to provide a complete assessment of intracellular signaling mechanisms that control intestinal epithelial proliferation and to provide clinically relevant information regarding stimulation of intestinal growth during periods of gut disuse, after small bowel resection, or with administration of chemotherapeutic agents.